In order to register with a network or to be re-registered with a new base station for a handover in a wireless access system, a mobile station (MS) must perform uplink synchronization with the base station (BS) and receive a management access ID. This process is referred to as initial ranging. In addition, the mobile station measures signal quality by receiving signals from neighboring base stations in order to search for an optimum base station for handover. This second process is referred to as scanning.
After scanning, the mobile station requests a handover to a serving base station when a base station appears that has a signal quality above a certain level. When receiving a handover response signal from the serving base station, the mobile station performs initial ranging with a handover target base station so as to be re-registered with the handover target base station.
The mobile station re-uses a ranging parameter during actual handover by measuring signal quality of neighboring base stations and pre-performing initial ranging in a scanning process in order to reduce a handover time before the handover is actually performed. This method is referred to as an association or a pre-handover.
FIG. 1 is a diagram illustrating an association procedure in accordance with a related art.
Referring to FIG. 1, a serving base station (e.g., BS1) periodically includes channel information associated with neighboring base stations (e.g., BS2 and BS3) in a MAC management message (e.g., MOB-NBR-ADV) and sends the MOB-NBR-ADV to a mobile station (MS). At this time, the MOB-NBR-ADV is sent to the MS as a broadcast.
Upon receiving the MOB-NBR-ADV, the MS sends a scanning request message (e.g., MOB-SCN-REQ) including scanning duration information to the serving base station (e.g., BS1). The BS 1 includes information related to a scanning rendezvous time and the scanning duration in a scanning response message (e.g., MOB-SCN-RSP) and sends the MOB-SCN-RSP to the MS.
Upon reaching the scanning rendezvous time, the MS performs the scanning by performing downlink synchronization with neighboring base stations and measuring signal quality. During the scanning duration, the MS performs an association by performing initial ranging with each base station. Much time may be required to perform the initial ranging because radio resources for the initial ranging are allocated based on contention.
Upon expiration of the scanning duration, the MS disconnects from the neighboring base stations and performs uplink and downlink synchronizations with the serving base station BS1, thereby allowing transmission and reception of data to and from the serving base station.
However, the association procedure of the related art has several problems. First, since the association is performed during the scanning process, there is no standard for which of the base stations is the one with which the mobile station is to associate. Thus, the mobile station performs associations with all neighboring base stations, thereby taking much time for handover. Second, it may take longer to perform the related art association and there is difficulty in setting the scanning duration at a scanning request because contention based initial ranging is performed. Furthermore, if the scanning duration is short, the association may not be performed within the scanning duration. If the scanning duration is excessively long, downlink traffic which should be transmitted to the mobile station for the scanning duration stays in the base station. Furthermore, waste of radio resources may result from these problems.